The present invention relates to the field of catalytic combustion, and provides a catalytic combustor that does not require continuous preheating
Catalytic combustors which are built up from metal foil that is coated with catalyst on just one side have been described in the prior art, for example, in U.S. Pat. Nos. 5,202,303, 5,250,489, and 5,512,250, the disclosures of which are hereby incorporated by reference. An object of the catalytic combustion is to minimize the production of nitrogen oxides.
The combustors described in the above-cited patents were intended for gas turbines wherein the turbine compresses the incoming fuel-air mixture, thereby preheating that mixture. The patents show a catalyst coating on only one side of the foil. When the strip of foil is folded back and forth upon itself, the resulting structure comprises a combustor having channels which are catalyzed and channels which contain no catalyst.
Unlike the case of a gas turbine, the fuel-air mixture that enters a catalytic combustor in a home heating appliance such as a gas furnace is not preheated. If the catalyzed channels and the bare channels of the catalytic combustor are of the same size, there is so much cooling that the catalyzed surface cannot be kept hot enough to support combustion. The latter statement is true because of heat transfer from the catalyzed channels to the non-catalyzed channels. This heat transfer effectively cools the catalyzed channels below the temperature at which they will sustain combustion. The latter effect is observed even if the entering fuel mixture is preheated enough to ignite the combustion. When the preheating is stopped, the combustor cools and combustion stops.
The present invention solves the above-described problem by providing a catalytic combustor which can operate without preheat. In particular, an appliance using a combustor made according to the present invention can be heated electrically to start the combustion, after which it will operate with no electric current.
The catalytic combustor of the present invention maintains combustion without preheat once the combustion has started. The combustor comprises channels which are coated with catalyst, and channels having no catalyst coating. The ratio of the cross-sectional area of the catalyzed channels to the cross-sectional area of the uncatalyzed channels is chosen to be great enough to support combustion without preheat, but low enough to prevent deactivation of the catalyst due to excessive heat of combustion. By making the cross-sectional area of the catalyzed channels larger than that of the uncatalyzed channels, the gas flow through the former will be greater than the flow through the latter, and the temperature of the dividing walls between the channels will be maintained at a temperature high enough to support catalytic combustion without preheat.
In one embodiment, the combustor of the present invention comprises a plurality of spaced-apart metal strips, the strips defining alternating wide and narrow channels for gas flow. Only the sides of the strips facing the wide channels are coated with catalyst.
Another embodiment comprises a plurality of corrugated strips, separated by a plurality of pairs of flat divider strips, and a plurality of structural members located between the pairs of divider strips. The corrugated strips are coated with a combustion catalyst, and the structural members have no catalyst coating. The sides of the divider strips which face the corrugated strips are also coated with catalyst.
In another embodiment, the catalytic combustor comprises a continuous strip of metal which is folded back and forth upon itself, with a plurality of corrugated strips positioned within some of the folds, and a plurality of structural members positioned within other folds, the corrugated strips and structural members alternating with each other. Only the side of the continuous strip which faces the corrugated strips is coated with a combustion catalyst. The continuous strip is connected to a source of electric current, and preferably has an electrically insulating coating.
Another embodiment comprises a stack including a first flat strip, a first corrugated strip, a second flat strip, and a second corrugated strip, the stack being wound into a spiral. The first corrugated strip has corrugations which are larger than the corrugations of the second strip. At least the first corrugated strip is coated with a catalyst. At least one of the flat strips is connected to a source of electric current. The flat strips preferably also have an electrically insulating coating.
In another embodiment, the catalytic combustor comprises a continuous corrugated strip, the strip having wide and narrow corrugations which alternate with each other, the corrugated strip being folded back and forth upon itself. A plurality of divider strips separate the folds of the corrugated strip. The corrugated strip is coated with catalyst on the side which defines the wide corrugations, and is connected to a source of electric current. The divider strips preferably have an electrically insulating coating. The flat strips may also be coated with catalyst on the sides which face the wide corrugations.
In another embodiment, the catalytic combustor comprises first and second continuous strips which are wound together to form a spiral. The first strip has alternating wide and narrow corrugations, and the second strip is substantially flat. The first strip is coated with a catalyst on the side which faces the wide corrugations. The second strip is also coated with catalyst on the side which faces the wide corrugations. At least one of the strips is connected to a source of electric current, and at least one of the strips has an electrically insulating barrier.
In another embodiment, a plurality of strips are arranged to define alternating wide and narrow channels. The sides of the strips that face the wider channels are coated with catalyst, and the other sides are not coated. The strips are positioned such that the size of the channels tapers down in the direction of gas flow. This embodiment can be constructed using a single strip which is folded back and forth upon itself to define the various channels.
The invention also comprises a method of operating a catalytic combustor. The method includes the step of selecting a ratio of the cross-sectional area of catalyzed channels, to the cross-sectional area of the uncatalyzed channels, such that the ratio is large enough to sustain catalytic combustion, and small enough to prevent deactivation of the catalyst due to excessive heat of combustion. Then, a combustion gas is passed through the combustor, without any preheat. The method may also include the step of igniting the combustion gas by non-catalytic means, such as by electric heating, and halting the ignition step when the combustion gas has ignited. If the above-mentioned ratio is properly chosen, the combustor will operate continuously without preheat.
The present invention therefore has a principal object of providing a catalytic combustor which, once ignited, requires no preheat.
The invention has the further object of providing a method of operating a catalytic combustor, which method does not require preheat.
The invention has the further object of providing a catalytic combustor which can be ignited by resistive heating.
The invention has the further object of providing a method of catalytic combustion in which the combustion can be started by resistive heating.
The invention has the further object of providing a catalytic combustor which is compact, so that it can be used in home heating appliances.
The invention has the further object of providing a catalytic combustor for use in a gas turbine.
The reader skilled in the art will recognize other objects and advantages of the present invention, from a reading of the following brief description of the drawings, the detailed description of the invention, and the appended claims.